A metal detector is known as a device for applying an alternate magnetic field to examine the response characteristics. The metal detector comprises a search coil that generates an alternate magnetic field to generate an eddy current on a metal surface of the test object, which in turn generates a magnetic field repulsive to the applied magnetic field. Since this magnetic field changes an electromotive force in response to changes in the magnetic flux penetrating through the search coil, as may be represented using Faraday's law of electromagnetic induction, existence of metal can be detected by measuring the changes in a signal. Also known are methods of nondestructive inspection such as a method for inspecting a defect in a steel pipe or a wire rope by generating an eddy current in the same manner as in the case of the metal detector. Among applied examples of the metal detector are a metal detection gateway for preventing dangerous articles from being carried in for the purpose of terrorism prevention or crime prevention and a device for detecting a piece of metal, a needle for example, having been misplaced during manufacture in articles such as meat or clothes.
As a method for testing electrical characteristics of a living body, there is the bioelectrical impedance method in which an electrode is attached on the skin to apply a feeble alternate current to measure the impedance thereof. The method is most commonly used in body fat meters. Meanwhile, a method of applying, instead of applying current from an electrode, an alternate magnetic field to a human body to generate an induced current which in turn is detected by means of a search coil is disclosed as the magnetic bioelectrical impedance method in A Noninvasive Electromagnetic Conductivity Sensor for Biomedical Applications by Lynn W. Hart, et al., IEEE Transactions on Biomedical Engineering, Vol. 35, No. 12 (1988) pp. 1011-1021 (nonpatent reference 1).
Nonpatent reference 1: A Noninvasive Electromagnetic Conductivity Sensor for Biomedical Applications by Lynn W. Hart, et al., IEEE Transactions on Biomedical Engineering, Vol. 35, No. 12 (1988) pp. 1011-1021
In any of these detection methods, use is made of such search coils that capture the component of a magnetic field in the direction vertical to the apply coil face. Accordingly, it is only a change of a magnetic characteristic caused by a change in the induced current due to the material characteristic that can be captured during detection.
In Japanese Patent Application Laid-Open No. 2003-199723 (patent reference 1), there is disclosed a method for estimating the current density or the electric conductivity of a conductive material, in which an induced current is generated with a magnetic field or a current is directly applied, and also disclosed is a method for making an analysis with the use of a vectorial magnetic sensor.
Patent reference 1: Japanese Patent Application Laid-Open No. 2003-199723
As a method for nondestructive inspection for inspecting a defect in a metal material, there is known a method of generating an eddy current and measuring a magnetic field generated therefrom by means of a detector coil, the method being referred to as, for example, eddy current flaw detection. The detector coil detects as a component of the magnetic field to be measured the component parallel to the central axis of the magnetic field apply coil. In Japanese Patent Application Laid-Open No. H5-203629 (patent reference 2), it is disclosed that use is made of, as a detector coil placed inside the apply coil, a coil that detects the component of the magnetic field in the direction vertical to the central axis, since the impedance of the detector coil may be influenced, for example, by the material characteristic of the test object and the distance.
Patent reference 2: Japanese Patent Application Laid-Open No. H5-203629
A method of measuring the current distribution for the purpose of inspecting a defect in metal or plate materials made of carbon fiber is disclosed in Non-contact SQUID-NDT method using a ferrite core for carbon-fibre composites by Y. Hatsukade, et al., Superconductor Science and Technology, Vol. 15 (2002) pp. 1728-1732 (nonpatent reference 2). In this method, opponent polarities are applied onto the test object at a distance from each other to apply a magnetic field and a current is strongly induced in the test object between the opponent poles. As for the direction of a component of the magnetic field to be measured, measurement is made by means of a superconducting quantum interference device (SQUID) as a magnetic sensor for a difference in the same direction as that of an applied magnetic field and in the direction perpendicular thereto. Thus, an induced current is generated only in proximity to the magnetic sensor, and an image is created by synthesizing the currents having been measured at different measuring points on the test object.
Nonpatent reference 2: Non-contact SQUID-NDT method using a ferrite core for carbon-fibre composites by Y. Hatsukade, et al., Superconductor Science and Technology, Vol. 15 (2002) pp. 1728-1732
A method is disclosed in Two-Dimensional Mapping of Impedance Magnetocardiograms by A. Kandori, et al., IEEE Transactions on Biomedial Engineering, Vol. 49, No. 7 (2002) pp. 721-728 (nonpatent reference 3), in which an electrode is directly attached to a living body to apply a current and a magnetic field generated therefrom is measured by means of a SQUID. In this measurement, there is provided a coil for detecting a component of the magnetic field in the direction perpendicular with respect to the test object, and a component of the magnetic field similar to that in the case of metal detection or the magnetic bioelectrical impedance method is detected. Here, because there are magnetic fields entering into the detector coil as well as the magnetic field from the living body, the detector coil is provided with a cancel coil.
Nonpatent reference 3: Two-Dimensional Mapping of Impedance Magnetocardiograms by A. Kandori, et al., IEEE Transactions on Biomedial Engineering, Vol. 49, No. 7 (2002) pp. 721-728
Further, there is disclosed a method for inspecting the current distribution in a human body in Multichannel SQUID system detecting tangential components of the cardiac magnetic field by K. Tsukada, et al., Review of Scientific Instruments, Vol. 66, No. 10 (1995) pp. 5085-5091 (nonpatent reference 4), in which an image of the current distribution can be created by detecting each of orthogonal x and y components of the magnetic field, the xy plane being parallel to the body surface. In this method, however, it is a current autonomously flowing due to electrophysiological phenomena in the cardiac muscle that is to be measured and a current is not induced in the living body, and a change of the electrical impedance of a living body cannot be measured.
Nonpatent reference 4: Multichannel SQUID system detecting tangential components of the cardiac magnetic field by K. Tsukada, et al., Review of Scientific Instruments, Vol. 66, No. 10 (1995) pp. 5085-5091